1. Field of the Invention:
This invention relates to an ultrasonic diagnostic apparatus for use in the imaging or characterization of biological tissue.
2. Description of the Prior Art:
Ultrasonic imaging systems are at present well known. The most widely used technique is the pulse echo B-Scan mode, or variants thereof, in which a video display employs intensity modulation to depict echo return amplitude as a function of depth.
The echoes are generated at interfaces between media of different acoustic impedance within the specimen tissue. However, the magnitude of the interfacial reflection coefficient depends not only on the impedance ratio, but also on the angle of incidence of the beam, and furthermore the echo amplitude is reduced due to attenuation from the intervening tissue. Thus, the images generated by B-Scan systems illustrate the geometry of the interfaces, which can be interpreted qualitatively but do not give quantitative information on bulk tissue due to geometric distortion of the echo amplitude arising from refraction, specular reflection, and diffraction. Therefore, it is difficult to distinguish between tissue states.
Currently in medical diagnostic ultrasound equipment, measurements of the sound speed, attenuation and backscattering are made to assist the clinician by increasing the accuracy and subtlety by which it is possible to differentiate structure and/or function in a particular region of the human body.
Particularly, general information on measurement of the acoustic attenuation coefficient is given in a magazine article entitled "Estimating the Acoustic Attenuation Coefficient Slope for Liver from Reflected Ultrasound Signals" in IEEE Transaction on Sonics and Ultrasonics, Sept. 1979, Vol. SU-26, No. 5, pp. 353-362.